1. Field of the Invention
The present invention relates generally to rotating disk drives and, more particularly, to a disk drive having a write condition detector that generates a write condition signal to indicate that the writing of data in a user data segment during a write operation is unsafe when a fly-height-representing signal exceeds a threshold.
2. Description of the Prior Art
The competitive nature of the disk drive industry encourages rapid technological innovation. One result of this innovation is the advent of the low-flying head. A low-flying head enables an increase in the areal recording density, expressed in bits per square inch, that enables smaller, yet higher capacity, disk drives. Refer now to FIG. 1 which shows a modern low-flying head 10. Head 10 writes data to and reads data from a disk 12. The disk spins in the direction of arrow 4. Head 10 comprises a slider 78 and a transducer 76 constructed in a conventional manner. Head 10 `flies` above the spinning disk 12 on an air bearing created by the relative motion between the disk recording surface 89 and head 10. The part of the head 10 in contact with the air bearing is known as the air bearing surface 86. Head 10 is fixed to a suspension arm (not shown) that can drive the head 10 across the disk recording surface 89 in response to an actuator system. The suspension also exerts a force on head 10 that helps keep the head flying close to the disk recording surface 89. The distance between the air bearing surface 86 and the disk recording surface 89 is known in the industry as fly height 9.
Just a few years ago heads were designed with a positive pressure air bearing across the entire slider. In a positive pressure air bearing, the air bearing surface is designed to create pressure across the surface of the bearing. In comparison to modern heads, these old heads flew relatively high, 2 to 3 microinches, above the spinning disk surface. Modern heads fly much closer to the spinning disk surface, at approximately 1.2 microinches, using a negative pressure zone along a portion of the slider with positive pressure air bearings. The negative pressure zone creates low pressure under part of the head 10 that helps keep the head close to the spinning disk recording surface 89 using the Venturi effect. Typically, the middle portion 84 of the head creates the low pressure zone and the outside portions 83 and 85 of the head create the high pressure air bearings. This is analogous to a modern race car incorporating a `ground effect` where the wheels support the car off the ground and the body of the car is shaped to create a low pressure zone under the car holding the car to the road. In the analogy, the high pressure air bearings are the wheels, the disk is the road, and the low pressure zone of the slider is the body of the car.
For each combination of disk surface 89 and head 10, there is defined a nominal fly height 82, a high-fly threshold 81 and a low-fly threshold 91. Flying higher than the high-fly threshold 81 or flying lower than the low-fly threshold 91 during normal disk drive operations can lead to data errors or to other consequences, such as a head crash. The high-fly threshold 81 and the low-fly threshold 91 define the operating fly height range of head 10.
Because modern heads fly so close to the spinning disk surface, contaminants or defects in the disk recording surface 89 may cause head 10 to fly high out of the operating fly height range. Contaminants such as particles or debris can become stuck to, and caught under, head 10 and cause the head to fly high. Also extreme temperature and pressure variations may cause the head to fly low.
Head 10 is shown with contaminants 77 stuck to its trailing edge 87. Often contaminants can be smeared along the trailing edge 87 of the air bearing surface on the bottom of a head. The trailing edge 87 of the air bearing surface is the location of the magnetic recording elements.
Currently, the industry has identified certain sources of contamination such as excess fluorocarbon lubrication, hydrocarbons condensed onto the head, carbon from the disk overcoat that has been burnished off, and particulate contamination debris. The industry uses fluorocarbon lubrication to prevent the head from sticking during starts and stops. Hydrocarbons may condense on the head from oil on disk drive parts, adhesives, and grease from disk drive bearings. Disk carbon originates from carbon that has been burnished from the carbon overcoat of the disk by contact with the head. Other sources of contamination and debris are environmental such as dust and smoke particles.
A high or low-flying head can cause many problems in a disk drive. Data errors may be caused by a high-flying head during a write operation, known to those in the industry as a `high-fly write.` These data errors may be either soft errors that are correctable by the disk drive's error recovery systems, or these data errors may be hard errors that are impossible to correct with the disk drive's error recovery systems. Even though a `low-fly write` is not considered a source of data errors per se, because the closer the head is to the disk the better the data is written, a low-fly write may nevertheless be an indicator of an impending head crash. In either case, it would be desirable to monitor the condition of the write operation to know when a high-fly write or low-fly write has occurred.
Because modern heads fly relatively close to the disk surface, a high-fly will have a much more dramatic effect on the ability of the head to write the disk. For example, a head that flies nominally at 3 microinches experiences a 1 microinch increase in fly height to 4 microinches due to contaminants sticking to the head. The change in fly height is 33.3% of the nominal fly height. In contrast, if a head that flies nominally at 1.2 microinches experiences the same 1 microinch increase in fly height to 2.2 microinches the change in fly height is now 83.3% of the nominal fly height.
Because old style heads fly relatively high above the disk surface they are relatively more susceptible to crashing into the disk surface. In contrast, very little except extreme pressure and temperature variations can make the modern head fly low. In some relatively rare cases, torsion of the head caused by contaminants asymmetrically affecting the head will cause one side of the head to rise and the other side to fall, resulting in a low-flying head. Generally, with a negative pressure zone head there is little change in fly height with changes in air pressure.
Historically, the art has focused on gauging fly height for the prediction of a head crash during a test at build time or the control of fly height using a head positioning system. As noted above, high-fly writes were not as significant a problem because the head was flying relatively high, as a result high-fly writes and low-fly writes are not addressed by the prior art.
Fundamentally, the art depends on the Wallace equation which expresses the dependence of the readback voltage on various parameters, such as head/disk spacing. In U.S. Pat. No. 4,777,544 to Brown et al., a system is described that computes the head/disk spacing by first recording a periodic signal on the disk at a predetermined location in a data track and measuring the readback signal at a first velocity and then again at zero velocity. In addition, a measure of relative change in head/disk spacing is obtained by the "Harmonic Ratio Flyheight" (HRF) method. One of the drawbacks of this system is that a portion of the disk surface is occupied by the periodic signal, thus reducing the effective user data storage capacity. Brown et al. do not address the problem of high-fly writes or low-fly writes.
One system that utilizes the HRF method of U.S. Pat. No. 4,777,544 is described in U.S. Pat. No. 5,377,058 to Good et al. This system dynamically adjusts the fly height of a head using a piezoelectric element. One drawback of this system is a dependence on a dedicated fly height signal to control the head fly height and the attendant reduction in user data storage space in a data track. Like Brown et al, Good et al. do not address the problem of high-fly writes or low-fly writes. Good et al. also has the additional drawback that head positioning only occurs during a read operation because the head reads the periodic signal from the data track as an integral part of the control system. Good et al. can not effectively control the head position during a write operation because the control system is blinded during the write operation. In other words, Good et al. would be just as detrimentally affected by a high-fly write or a low-fly write as any other system of the prior art.
U.S. Pat. No. 5,410,439 to Egbert et al. describes a system that uses the HRF method described above to predict a head crash. As with the other systems this system has the disadvantage of dedicating a portion of the hard disk storage space for the dedicated signal. Like Brown et al. and Good et al., Egbert et al. do not address the problem of high-fly writes or low-fly writes. Egbert et al. also has the additional drawback that for most of the predictive tests the disk velocity must be spun down to zero in order to make predictive measurements, making the tests unsuitable for use during normal write operations.
Accordingly, there is a need for a disk drive that reduces the detrimental effects of a high-fly write or a low-fly write in a user data segment.